I finally got around to playing it and man, this has to be the best F2P mobile game I've played. Beautiful art and music, fast, engaging, simple and challenging gameplay...
Also, the story doesn't interrupt at all (and I like the way it's told), the mere fact of collecting characters is pretty satisfying and the pay-to-win elements do not affect you at all: unless you blow out your morale in the Metal Zone, you should be fine and play for a long time before you reach zero (I haven't even spent all of them yet).
My team is: Bahl, Kuscah (healing is such a cool concept in this game), Manmer, Sha'plar, A'merpact and a lion king (I don't know how they're called in English, in Spanish it''d be something like Fang Lion King).
So, three B's, one S and two A's. I also have Zeera (S-class), but she hasn't proved to be very useful to me. I guess I'll level her up in the Metal Zone even if she's already one of the members with a higher level.